Creating an ultra wideband radar system is important for all the above applications where high range resolution is required. There are a number of possible approaches to produce an extremely wide bandwidth for the transmission and reception circuits of a radar system. One approach is impulse radars as described in U.S. Pat. No. 6,091,354, U.S. Pat. No. 6,067,040 and S. Koppenjan et al., “Multi-frequency synthetic-aperture imaging with a lightweight ground penetrating radar system”, Journal of Applied Geophysics 43, pp. 251-258, 2000. An alternate approach is to use a stepped frequency radar as described in U.S. Pat. No. 6,664,914.
However both these approaches have severe limitations. Impulse systems only receive a small percentage of the signal, so high transmission powers or long integration times are required to produce a high quality signal. Stepped Frequency or Frequency Modulated Continuous Wave (FMCW), is more efficient, but is not allowed under current US Federal Communications Commission (FCC) regulations as it has a narrow band transmission at any point in time.
Ideally, a wideband radar system would generate a signal that covers the entire radio spectrum eg from DC to 5 GHz. A receiver would then record the returned waveform at more than double the highest frequency in the transmission eg >10 Gsps. To enable the detection of both strong and weak signals, it is desirable to have a low digital quantisation noise and thus a high bit resolution (12-16 bits) Analog to Digital Converter (ADC) would be required. Gain control may also be required to allow a larger dynamic range as the signal gets weaker. However currently there are no 12 bit ADCs with sample rates >10 Gsps. At lower sample rates, ADCs do exist, but they are expensive and the digital control logic required to process the incoming data also increases the cost and complexity.
One approach to digitizing the signal is to under-sample the received signal. This is demonstrated in J. Sacks et al., “Integrated Digital UWB-Radar”, AMEREM 2002, 2-7 Jun. 2002, Annapolis, Md., where the transmission of a high frequency signal is repeated a number of times, each time sampling different elements of the returned signal. The problem is that as the receiver is not always active, the signal to noise performance of the system is reduced. However the reduction is not as severe as it would be with an impulse system.
Another approach for producing a wideband receiver is to reduce the number of bits in the ADC. This is because as the number of bits in the receiver reduces the complexity also reduces, thus allowing the speed to increase. The limit of this reduction is to simplify the ADC to a single bit or to a comparator. This allows the received signal to be continuously monitored however the output is only one bit. One example using a single bit ADC in a radar is the random noise automotive radar system suggested in U.S. Pat. No. 6,121,915 and U.S. Pat. No. 6,392,585 B2. This system uses a random noise source as the transmitter. The receiver uses two comparators, one sampling the transmitted signal and the other sampling the received signal. By cross correlating these two waveforms, a range profile can be produced. Unfortunately, this approach has limited range and dynamic response due to the size of the cross correlation array. It also requires the design of a large and complicated Application Specific Integrated Circuit (ASIC) chip to achieve the cross correlation in real time.